This invention relates to a circuit arrangement for switching between different current values by means of mechanical or electronic switches, more particularly in terminal equipment and remote control units of teletypewriter and data transmission systems connected to exchanges over single-current lines.
If the subscribers in teletypewriter and data transmission systems are connected to exchanges over single-current lines, it is necessary to switch the teletypewriter and terminal equipment between four different current values, e.g., the current values 0 mA, 2 mA, 5 mA, and 40 mA. As generally known, the values 0 mA and 40 mA are assigned to the call-connection condition, the value 5 mA to the idle condition, and the value 2 mA to the connection-release condition.
The switching referenced above heretofore occurred by means of three mechanical switches. FIG. 1 shows such a circuit arrangement in which on the left-hand side the terminal equipment is shown schematically with the remote control unit, in the middle the line L, and on the right-hand side the exchange V having the polarity-reversing means.
The current value 40 mA can be set by means of the switch K1, and the current value 5 mA or 2 mA by means of the switches K2, K3.
In the electronic circuits for remote control units proposed heretofore, the mechanical switches have not simply been replaced by electronic switches, because the necessary expense appeared excessive. Not only three electronic switches (e.g. transistors) are necessary, but three additional coupling circuits must be used, because of the resistive decoupling required in most cases. For this reason, in prior art electronic circuits, no distinction was made either between the current values 5 and 2 mA or 2 and 0 mA, or the two values were assigned to the different polarity of the line during the call setup and the clearing of a connection. Such an arrangement is shown in FIG. 2. In the Figure one of the resistors of the branch controlled by switch S2 is bridged by a diode, so that a current value of 5 mA results in one direction of current flow and a current value of 2 mA in the other direction of current flow. In this arrangement, only two electronic switches are required. However, since the electronic switches are polarity-dependent, both switches must, for example, be switched into the diagonal of a bridge rectifier.
These two solutions applied to electronic circuits of known construction have operational disadvantages. Thus, for example, the lack of significant distinction between 0 mA and 2 mA leads to distortions during the transmission of the teletypewriter signals; the lack of significant distinction between 2 and 5 mA leads to limited reliability in the case of a connection release. However, if the two values are assigned to the different polarities of the line during call setup and connection clearing, call cancellation during the call setup is in certain circumstances impossible, because during the call setup the polarity of the line that prevails during the idle condition is still available. To avoid these drawbacks, all occurring current values should be adjustable. However, also the number of required switches should be held to a minimum.
An object of the invention, therefore, is to provide a means for switching between different current values with a minimum number of switches using either mechanical or electronic switches, while avoiding operational disadvantages.